The demands on braking systems are increasing. This applies in particular to reliability and good fall-back level. If the brake booster fails, an internationally specified foot force of 500 N should achieve a delay of more than 0.64 g, which is significantly greater than the minimum legislated requirement of 0.24. An advantage of the high achievable deceleration is that, if the brake booster fails, a red warning lamp that irritates the driver does not need to be triggered.
These requirements can be solved by means of brake-by-wire systems with a path simulator. Here, the master cylinder (Hz) or tandem master cylinder (THZ) is designed for the fall-back level in case of a braking system failure. This is achieved through appropriate dimensioning with a small diameter. This creates higher pressures at a corresponding foot force. The required brake fluid volume for 0.64 g and corresponding pressure is relatively small compared to that at maximum pressure at full vehicle deceleration and fading. A THZ cannot fully raise the necessary volume, even with a larger stroke. The applicant's document DE 10 2009 043 494 proposes a solution for this involving a storage chamber that, at higher pressures, feeds corresponding volumes into the brake circuit. A further solution is described in the applicant's document DE 10 2010 045 617 A1, in which volumes from the master cylinder are delivered from the reservoir into the brake circuit via the corresponding valve and THZ control. In the case of vehicles with large volume intakes, e.g. SUVs and vans, brake circuits must be filled up during deceleration even before the blocking pressure for high μ. Both solutions place high demands on the tightness of the valves.
Furthermore, interruption of pressure build-up and small brake losses are related to the additional filling of the brake circuits.
The applicant's document DE 10 2011 111 369 describes a system with an additional piston, which achieves the required pressure fluid volume and has the advantage of being actuated by the motor spindle and ineffective in the fall-back level, allowing the predetermined deceleration. Under certain circumstances, the correspondingly high forces could negatively affect the spindle, ball screw drive (KGT) and the bearings.
Another important aspect is the installation length. There are two different types of braking systems, “serial design” S and “parallel design” P (also referred to as “S-system” or “P-system”). This means that, in the case of the S-system, the main components (e.g. in DE 10 2011 111 369) of the master cylinder THZ, motor with the ball screw drive KGT and the auxiliary pistons are arranged in one axle, and in the P-system (e.g. in DE 10 2012 222 897 A1), the master cylinder THZ is arranged in one axle, while a piston cylinder unit (plunger) for volume supply is arranged in a laterally offset second axle with a motor.
P-systems require less installation length, but are more complex and differ from the S-system in reliability.
According to the applicant's document DE 10 2013 111 974.3, a P-system with double-stroke piston and THZ is achieved with an installation length and valve circuit that does not yet meet all requirements.
A brake system for motor vehicles in which a driven double-stroke piston is used to build up brake pressure in the wheel brakes is known from WO 2012/017037 and WO 2011/157347. The common feature of both applications is that the delivery volume is fed to the brake circuits via a single-circuit connection line from the double-stroke piston via feed valves. However, an alternative separate supply to the brake circuits via feed valves includes a connecting valve that merges the brake circuits. Both solutions are error-prone because, in the event of failure of the brake circuit and feed valves, failure of braking force amplification or total brake failure follows. DE 2006 030 141 also plans for a single-circuit connection line, which is connected to loads via changeover valves. These valves have an additional connection to the reservoir. The single-circuit connection with changeover valves is also error-prone here when safety-relevant loads are used, e.g. brake or transmission systems.